Site-specific electronic and geometric interface structure of Co-tetraphenyl-porphyrin layers on Ag(111)

We present a combined multimethod experimental and theoretical study of the geometric and electronic properties of Co-tetraphenyl-porphyrin (Co-TPP) molecules adsorbed on a Ag(111) surface. Scanning tunneling microscopy (STM) topographs reveal that Co-TPP forms highly regular arrays with a square unit cell. Hereby, the Co-TPP molecules do not occupy a unique adsorption site on the Ag(111) atomic lattice. The central Co atom of the Co-TPP is found to reside predominantly above fcc and hcp hollow sites of the substrate, as determined from the photoelectron diffraction patterns. A strong adsorption-induced deformation of Co-TPP involving a saddle-shaped macrocycle is evidenced by high-resolution STM images and quantified by near-edge x-ray absorption fine-structure measurements. By scanning tunneling spectroscopy we resolved discrete molecular electronic states and mapped the pertaining spatial charge-density distribution. Specifically, we discuss the interaction of orbitals originating from the Co-metal center with the porphyrin macrocycle and show that the varying adsorption sites induce a modulation in the Co-TPP lowest unoccupied molecular orbital. These findings are corroborated by density-functional-theory calculations.

Figure 1

(Color online) Co-TPP assembly on Ag(111): (a) large-scale topographic STM image. Every protrusion corresponds to a Co-TPP molecule ($V_b=-0.48\text{V}$ and $I=97\text{pA}$). The red square marks the unit cell. (b) Detailed image showing submolecular resolution. Besides the molecular main axis (three aligned maxima, dashed line) four dimmer protrusions corresponding to the phenyl groups can be identified ($V_b=-0.9\text{V}$ and $I=0.12\text{nA}$). The molecular packing is described by a square unit cell with side length $a=14.05\pm 0.2\text{Å}$. (c) Structural model of the unit cell. (d) Superposition of the Ag(111) atomic lattice ($V_b=-13\text{mV}$ and $I=4.9\text{nA}$) and the Co positions (red dots) in a Co-TPP array extracted from an STM image. This figure highlights the azimuthal orientation of the Co-TPP domain depicted in (b) relative to the Ag(111) atomic lattice (nearest-neighbor distance $a_0=2.89\text{Å}$) and indicates the higher-order coincidence structure of the overlayer (see text for discussion).

Figure 2

(Color online) RHEED patterns (15 keV) taken after Co-TPP deposition at room temperature and postannealing to 525 K. (a) The 1/4 spots point to a superstructure with a fourfold substrate periodicity along the $\left[\overline{1}\overline{1}2\right]$ direction of the Ag substrate. (0,0) indicates the spot of the reflected specular beam. (b) The 1/7 spots of the superstructure along the $\left[\overline{1}10\right]$ direction.

Figure 3

(Color online) (a) $\text{Co}2p_{3/2}$ x-ray photoelectron spectrum on Co-TPP/Ag(111) taken at a coverage of roughly three monolayers (dots) together with the best fit based on three Gaussian functions and a linear background (see text for details). (b) Plot of the 2D PED pattern of the measured anisotropy of the $\text{Co}2p_{3/2}$ emission (kinetic energy of 262 eV) from the Co(0) peak. The experimental polar range extends from $0°$ to $68°$ with respect to the surface normal. The blue lines highlight the main features. The Kikuchi lines are marked by the blue band. (c) Simulated anisotropy $\chi$ for Co atoms in both fcc and hcp stacking sites and in substitutional sites in the first two layers of the underlying Ag representing the best fit (see text for discussion).

Figure 4

(Color online) Top and side views of the interface structures of the emitting Co atom (blue circle) placed a distance $d$ above a three layer Ag(111) slab (gray balls) for each model considered in the simulated PED anisotropies.

Figure 5

(Color online) Adsorption-induced deformation of Co-TPP. (a) Pseudo-three-dimensional rendering of a high-resolution STM image suggesting a saddle-shaped macrocycle distortion and alternately rotated phenyl groups ($V_b=-0.7\text{V}$ and $I=0.46\text{nA}$). The model represents the geometry consistent with the NEXAFS data displayed in (b). The scheme introduces the angles ${\rho }_{\text{macro}}$ and ${\theta }_{\text{phenyl}}$. (b) Carbon $K$ edge NEXAFS spectra of one monolayer Co-TPP on Ag(111) exhibiting strong dichroism. The different angle dependences of peaks A and B evidence molecular moieties with different orientation with respect to the substrate. (c) Deconvolution of the leading edge into spectral parts originating from the macrocycle (green) and the phenyl groups (blue). (d) The angular dependence of the two moieties yields a nonplanar macrocycle $\left({\rho }_{\text{macro}}=30°\right)$ and rotated phenyl groups $\left({\theta }_{\text{phenyl}}=45°\right)$.

Figure 6

(Color online) Molecular electronic states of Co-TPP/Ag(111). (a) Tunneling spectra of the bare Ag(111) surface (gray) and over a Co-TPP molecule (red). While the spectrum on Ag(111) only shows the steplike increase around $-67\text{meV}$ related to the onset of the surface state band, three spectral features marked by the colored bars can be easily recognized on Co-TPP (HOMO: $-0.6\text{V}$, red; LUMO: $\sim 1\text{V}$, blue; and $\text{LUMO}+1$: $\sim 2.2\text{V}$, turquoise). A faint signal around $-1.8\text{V}$ $\left(\text{HOMO}-1\right)$ can only be detected by directly comparing a spectra taken on the macrocycle and in the center of Co-TPP (inset). (b) Contrast change in STM topographs for tunneling at specific sample bias voltages as indicated by the colored bars. (c) Visualization of the spectral density of molecular orbitals by $dI/dV$ mapping. The colored bars indicate to which spectral feature in (a) the $dI/dV$ maps correspond. The ovals in the right panel mark the positions of the phenyl groups. (d) Tunneling spectra at three positions on a molecule as indicated in the inset (offset for clarity). These spectra confirm the laterally varying density of states presented in (c) (stabilized at $V_b=1\text{V}$ and $I=0.25\text{nA}$, see text for further discussion).

Figure 7

(Color online) Metal contribution to specific molecular electronic states. (a) $dI/dV$ map representing the density of states corresponding to the HOMO of Co-TPP taken on a mixed Co-TPP/${\text{H}}_2\text{-TPP}$ array represented in the topographic STM image in (b). The spectral electron density originates from Co states, as the corresponding signature is absent for the free base species. Nevertheless, the apparent intensity is peaked on two opposite pyrrole rings (see overlaid model). By contrast, the $\text{LUMO}+1$ orbital represented by the $dI/dV$ map in (c) localized on the phenyl groups is not affected by the metal center [(a)–(c): $I=0.6\text{nA}$]. (d) Calculated density of states projected on the nitrogen $p_{\text{z}}$ orbitals in ${\text{H}}_2\text{-TPP}$ and Co-TPP; only the latter shows the metal-related feature at $\sim 0.7\text{eV}$ below the Fermi level. (e) Density of states projected on different $d$ orbitals of the central Co atom. At the same energy where the peak on the macrocycle is observed [compare (d) and (a)], $d_{xz}$ and $d_{yz}$ states dominate. The lateral position of the molecules is close to hcp sites.

Figure 8

(Color online) Diagram illustrating the energy-level alignment discussed in the text. The left part represents one monolayer of Co-TPP on Ag(111). The right part provides a tentative comparison with Co-TPP/Au(111), Ref. 42, assuming an identical work-function change.

Figure 9

(Color online) Spatial variation in the LUMO for Co-TPP/Ag(111). (a) Topographic STM image of the edge of a Co-TPP array. The colored dots mark the positions where the spectra represented in (b) were taken. (b) Series of STS spectra taken on the meso-carbon position for 20 Co-TPP molecules. The LUMO feature exhibits a spatial variation, that is clearly visualized as a superstructure, or Moiré-like pattern, in the $dI/dV$ map of the LUMO displayed in (c) [(a) and (c): $V_b=1.1\text{V}$ and $I=0.17\text{nA}$]. The second molecule from the top of the line is defective. (d) Variation in the LUMO onset energy and nearly constant FWHM of the LUMO. (e) Calculated density of state projected on the nitrogen $p_{\text{z}}$ orbital for three different positions of the Co-TPP on Ag(111). The variation in the LUMO signature is reproduced by these calculations (see text for discussion).

Figure 10

(Color online) (a) Local view on a specific part of the LUMO superstructure by $dI/dV$ maps [compare Fig. 9c]. In some regions of the superstructure a clear checkerboard-type intensity variation is observed, where the two molecules on one diagonal of the unit cell are bright, while the two others appear dim $\left(I=0.6\text{nA}\right)$. Due to the shifted LUMO onset and the varying intensity, this contrast inverts in a relatively small-energy window corresponding to 200 mV. This checkerboard structure is related to the noncommensurate structure of the Co-TPP arrays (compare Fig. 1 and see text for further discussion). The white square marks the unit cell of the Co-TPP overlayer. (b) $dI/dV$ map on a larger area: the LUMO modulation does not constitute a regular checkerboard structure ($I=0.3\text{nA}$ and $V_b=1\text{V}$, low-pass filtered).